This invention relates to a method and system for increasing accuracy of clamping force of electric brakes of aircraft, and more particularly relates to a method and system for increasing accuracy of clamping force of electric aircraft carbon brakes providing greater accuracy for low brake clamping force commands by dedicating a portion of a plurality of electric brake actuators of each brake to low brake clamping force commands, without otherwise affecting normal braking.
Commercial aircraft commonly have landing gear with electrically actuated brakes for wheels mounted to the wing and body of the aircraft. The electrically actuated brakes are typically carbon brakes including a torque plate and a carbon heat sink stack containing the friction surfaces that are clamped together by four electric brake actuators with a clamping brake force to cause a wheel to decrease its speed of rotation. In such a conventional airplane carbon brake system, when braking is commanded, either by a pilot's actuation of a brake pedal or automatic braking, it causes the friction surfaces of the carbon brakes to make contact, creating brake torque to slow down the rotational speed of the wheel, and through contact with the ground, the taxi speed of airplane.
As is described in U.S. Pat. No. 7,441,844, it is possible to reduce brake wear of electrically operated aircraft carbon brakes, once braking has been commenced, by maintaining a minimum light residual clamping brake force when braking is no longer commanded, such as when a pilot stops pressing on a brake pedal, or otherwise during a commanded release of braking during automatic braking. During taxiing of commercial aircraft, particularly at low speeds, steering of the aircraft is typically controlled by braking, and an unequal distribution of brake energy due to inaccurate metering of brake clamping force can in some instances interfere with the directional stability of aircraft, particularly when a minimum light residual clamping brake force is maintained during taxiing when braking is no longer commanded. Unequal distribution of brake energy due to inaccurate metering of brake clamping force can also result in damage to wheels and brakes from exposure to excessively high temperatures. It has been found that it is not possible with currently available electrical braking systems to achieve a brake clamping force accuracy required by current industry standards for the Boeing 787, and to prevent unequal distribution of brake energy. What is therefore needed is a method and system for providing greater sensitivity to brake commands, particularly at low brake clamping force levels, without otherwise affecting normal braking. The present invention satisfies this and other needs.